Of the various types of turbines that have been developed over the years, there is included a type wherein a fluid such as steam or hot gas, for example, moves generally tangentially over the surface of rotatably mounted discs for imparting a rotational force to the discs by friction. This type of turbine is sometimes termed a "friction turbine" and is herein termed a "drag turbine". Early patents on this type of turbine include U.S. Pat. No. 1,061,206 to Tesla and No. 1,056,338 to Johnsen, and numerous subsequent patents have been issued for improvements in this type of turbine.
In general, a drag-type or friction-type turbine includes a pressure-tight, cylindrical casing surrounding a rotor comprised as a plurality of closely spaced discs that are usually flat and parallel to each other and commonly perpendicular to the concentric shaft to which they are attached. A driving fluid such as steam or the like is directed into the casing tangentially of the discs through one or more nozzles and then passes spirally inward between the discs and exits through apertures in the discs near the shaft or through a hollow shaft having apertures between the discs. During spiral passage between the discs, the fluid imparts a tangential force to each disc by virtue of the frictional shear set up at the disc walls, and this creates a torque on the rotor causing the rotor to spin. The ultimate angular velocity attained by the rotor is such that the foregoing torque is balanced by the combined effects of a load on the output shaft, frictional torque on the shaft bearings, and the torque due to windage loss between rotor and casing. In passage through the drag-type turbine, fluid transmits a part of the kinetic energy thereof and part of the momentum thereof to the discs and thence to the attached shaft and load. It is noted that in this type of turbine torque transmission occurs almost entirely by frictional drag, rather than by pressure or impact upon intercepting vanes or blades as in conventional turbines. The magnitude of the torque transmitted from a fluid to a rotor in a drag-type turbine increases with an increase in the relative tangential velocity between fluid and discs, and increases with an increase in the effective area of each disc. Additionally, the magnitude of torque transmitted increases with a decrease in the spacing between discs.
It is additionally noted that for a given velocity of entering fluid the magnitude of the power transmitted to the rotor varies with relative rotor speed, and passes through a maximum when the average absolute rotor speed is about one-half the average absolute tangential fluid speed between discs. Unfortunately, the efficiency of energy transfer is no more than fifty percent at the foregoing large differences in relative velocity between fluid and rotor. For highest energy transfer efficiency, it is desirable to minimize the relative velocity between fluid and discs, at all radial distances along the discs. From the foregoing, it is believed to be clear that it is possible in principle to achieve high efficiency by allowing the rotor to spin at a rate such that the rotor tip speed is only slightly lower than the speed of the entering fluid. However, practical limitations in the tensile strength of rotor materials limit the rotor tip speed. As an example employed further herein, the tip speed may be limited to the order of 1200 to 1500 feet per second. Consequently, a high efficiency drag turbine is limited to utilizing entering fluid speeds of this same order of magnitude.
In common with other types of turbines, the drag-type turbine of the present invention employs one or more nozzles for the purpose of converting the "pressure times volume" energy of the fluid into directed kinetic energy. For those applications wherein the initial pressure of the driving fluid is so low that it cannot be accelerated above the range of 1200 to 1500 feet per second by passage through a nozzle, there is no difficulty in attaining a high efficiency in a single stage drag turbine. This can be readily accomplished by providing adequate disc area and expanding all of the available pressure drop in a first stage, while at the same time allowing the rotor discs to spin fast enough to approach a match of disc tip speed and fluid inlet speed. For many applications, however, the initial pressure of the driving fluid is very high and, consequently, the only feasible way to achieve high efficiency is by multistaging or providing a multiplicity of stages of the drag turbine. Conventional staging of this type of turbine is suggested in the above-noted patent to Johnsen as providing one turbine following another, with the exhaust from the first being passed through nozzles for direction into a second in order to drive a common shaft. This type of multistaging requires separate turbine casings and substantial additional floor space. The present invention provides a highly advantageous alternative to conventional multistaging of drag turbines.